A 68-year-old man with benign prostatic hyperplasia (BPH) is being counselled about medical management options. His urologist explains the difference between 5-alpha reductase inhibitors and alpha-1 adrenergic antagonists. Which feature best distinguishes the mechanism of action of 5-alpha reductase inhibitors from alpha-1 blockers in BPH?

Explanation

## Distinguishing Mechanism: 5-Alpha Reductase Inhibitors vs Alpha-1 Blockers ### Mechanism of Action Comparison | Feature | 5-Alpha Reductase Inhibitors | Alpha-1 Blockers | |---------|------------------------------|------------------| | **Primary target** | Androgen metabolism (DHT production) | Smooth muscle α~1~-adrenergic receptors | | **Effect on prostate size** | Reduces volume (20–30% over 6–12 months) | No change in prostate volume | | **Onset of action** | Slow (3–6 months for symptom relief) | Rapid (days to 1–2 weeks) | | **Type of obstruction addressed** | Static obstruction (tissue hyperplasia) | Dynamic obstruction (smooth muscle tone) | | **Long-term benefit** | Prevents progression; reduces BPH-related events | Symptom relief only; no disease modification | **Key Point:** 5-alpha reductase inhibitors (finasteride, dutasteride) work by blocking conversion of testosterone to dihydrotestosterone (DHT), the hormone responsible for prostate epithelial growth. This leads to actual reduction in prostate volume — a structural change. **Key Point:** Alpha-1 blockers (tamsulosin, doxazosin) relax smooth muscle in the prostatic capsule and bladder neck, improving urine flow without changing prostate size. ### Why This Distinction Matters **High-Yield:** The only feature that truly distinguishes 5-alpha reductase inhibitors is their ability to reduce prostate volume. Alpha-1 blockers cannot shrink the prostate — they only improve symptoms by reducing outlet resistance. **Clinical Pearl:** In clinical practice, 5-alpha reductase inhibitors are preferred in men with large prostates (>40 g on ultrasound) or at risk of acute urinary retention, because they address the underlying pathology. Alpha-1 blockers are first-line for rapid symptom relief in men with smaller glands. **Mnemonic:** **5-ARI = Shrink; Alpha = Relax** — 5-alpha reductase inhibitors shrink the prostate; alpha-1 blockers relax the outlet. [cite:Harrison 21e Ch 297]